Hematocrit is conventionally determined by centrifugation or by a Coulter counter which determines the number and size of erythrocytes in a given volume of blood. Both methods provide high accuracy but cannot produce a continuous real time measurement of a patient's hematocrit.
It has previously been proposed in U.S. Pat. No. 4,243,883 to monitor the hematocrit of a flowing bloodstream by measuring the transmission of infrared light at a wavelength of 800 nm through the bloodstream. At that wavelength, transmission of light through the bloodstream is independent of oxygen saturation. However, the wavelength is extremely critical, and precision light sources of this wavelength are very costly and difficult to obtain. In addition, transmission is limited to a bloodstream thickness of a few millimeters, which impedes the blood flow and limits the blood volume which can be handled.
It has also been proposed to measure hematocrit by reflection, but this method presents sensitivity problems when used over a wide range of hematocrits.
As shown in U.S. Pat. No. 4,447,150, apparatus has previously been devised for optically determining the oxygen saturation of a flowing blood stream by measuring the reflection of certain wavelengths of light by a bloodstream. It has, however, not been possible heretofore to continuously and instantaneously measure the hematocrit of the blood stream without volume or flow restrictions. In addition, the use of optical methods for hematocrit determination can introduce errors as a result of temperature differences, optical noise and factors relating to the emission characteristics of the light sources used in the apparatus.